Decoding Liquids Before Your Very Eyes

Seeing really is believing. How often can we tell what a liquid is by just looking at it? Not too often. Sure, you might be able to tell when you definitely smell something sulfurous, or have a slippery base and I hope you can pick out milk. But we’re not always that lucky, especially if you’re dealing with something you really shouldn’t be touching or directly smelling. There are a ton of tests we can run to pinpoint what it is and you often need a pro to decipher the results. Ideally you could just read it with your eyes. With a batch of research from Harvard University, we’re one step closer.

The paper entitled, “Encoding Complex Wettability Patterns in Chemically Functionalized 3D Photonic Crystals” was featured in the August issue of the Journal of the American Chemical Society. The lead authors Ian B. Burgess and Joanna Aizenberg, of the Wyss Institute, propose a process to functionalize crystals so that they can differentiate between different fluids.


First, these aren’t just any of crystals. These are 3D porous photonic Inverse Opal Films (IOF). They were carefully created to maintain a specific structure. The ability to discern between different fluids is possible due to selective application and erasure of different chemicals. First, a functional group is applied to the surface of the IOF. A slab of PDMS (a silicon-based polymer) is sealed to an area of the IOF. O2 plasma is applied and erases the functional group except the area covered by the PDMS. This can then be repated with a second functional group, and so on and so on. There can even be overlapping areas covered to give you exactly what you need.


But what’s the point of functionalizing the surface? Well the functionalization affects the wettability of the IOF. Given the surface’s wettability and the surface tension of a liquid, the liquid won’t be able to penetrate the channels. There can be a clear change of color in the infiltrated regions. The authors refer to their system as Watermark-Ink (W-Ink) and suggest that it could be used as security measures. But I think I’d rather see it used in microfluidics. While you may toy with the notion that you could selectively choose which fluids flow through certain channels and partake in reactions, I think it is most useful as an indicator. Many microfluidic devices are intended to be used as point-of-care (POC) diagnostics. A sample is analyzed for a certain component that would indicate something about the patient's health. But at the end of the test, you have to be able to tell if the reactions were positive or negative. I think that the test could be designed to produce two fluids with different surface tensions, depending on the outcome. Then, one of two shapes would appear. It would require no confocal microscope or camera, and certainly wouldn’t need translation.


Burgess, I., Mishchenko, L., Hatton, B., Kolle, M., Lončar, M., & Aizenberg, J. (2011). Encoding Complex Wettability Patterns in Chemically Functionalized 3D Photonic Crystals Journal of the American Chemical Society, 133 (32), 12430-12432 DOI: 10.1021/ja2053013

Creating Droplets in Microfluidic Devices with Ultraviolet Light

Digital Microfluidics Background

With the widespread use of electronics, we often use the word ‘digital,’ but we might not always think about what it actually means. For those of you who have never taken a class in electrical engineering, or never learned Latin (from the word digitus), the word describes anything that is discrete as opposed to continuous. Digital has also been applied to a type of microfluidics. With the definition in hand, you might guess that digital microfluidics does not describe continuous fluid flow through channels at the micro- scale, but instead is made of droplets. Discrete droplets can be implemented in a variety of assays or devices, as they would allow for complete control of the fluid, instead of a continuous stream of fluid that may not be thoroughly mixed. Regardless of the intended use of the droplets, they must first be created.

There are currently three techniques to generate droplets:

  1. electrowetting
  2. dielectrophoresis
  3. emulsification

Electrowetting essentially uses an electric field to change how fluid interacts with the surface. It can make the surface more or less attracted to water, causing a fluid such as water to 'hug' the substrate or 'ball up' into a droplet. Manipulation of the electric field would provide control of the locations of droplets and how they move. Check out the videos from Dr. Richard Fair's laboratory at Duke that illustrate the formation and transportation of droplets using electrowetting.

Dielectrophoresis occurs when nonuniform electric fields cause polarizable particles to move. The application of dielectrophoresis for microfluidics was proposed by Dr. Thomas B Jones in the Journal of Applied Physics in 2001. Water is attracted to the regions where the electric field is the strongest. This movie from Jones does not demonstrate the formation of droplet formation, but it does illustrate its control over water.

Finally, the process of emulsification describes a system of two fluids in which one fluid is dispersed throughout the other. Think water and oil and the droplets you can create when you shake it around. This first requires at least two fluids to be used (I say at least two, because multiple emulsions can be achieved, as seen here) and an external stimulus. An external stimulus is often needed to cause stable droplet formation. This can occur at junctions, where specific geometry, along with control of flow rates can cause emulsification. You can see a video at the company RainDance's website. They have more information on the subject, and I recommend that you check out their other videos on that page, especially 'Loading droplets.' It's like a gumball machine!

UV Controlled Droplet Formation

While the ability to digitize fluid is valuable, its regulation can be increasingly more prized. Does the digitization have an on/off switch? Do you have to change the flow rates of the system to revert back to continuous flow, or physically move components that are responsible for pinching the droplets? The capability to switch between continuous and digital flow could serve to reduce the footprint of the device. Why make the system larger just to incorporate streams and droplets when it can happen in the same place? This would lend elegance to the design of the device. The sophistication would be improved if this could be accomplished without moving parts. The most reliable instruments and devices have fewer moving parts that could break down. This might be what Damien Baigl et al. from École Normale Supérieure in France had in mind. Their research, which was featured on the cover of the 2011 Issue 16 of Lab on a Chip, proposes a method to emulsify droplets with Ultraviolet (UV) light. Their paper entitled, "Photoreversible fragmentation of a liquid interface for micro-droplet generation by light actuation" describes an emulsification system that is controlled by the use of UV light.

To start, the system has water-in-oil flow. But the water contains a surfactant AzoTAB. When UV light is applied to this compound, a double covalent bond switches (from trans to cis). This causes the surfactant to become more polar and decrease the wettability with the surface of the device. The lowered wettability causes the water with AzoTAB to form droplets. Initially the researchers created a junction that was capable of emulsification depending on the flow rates of the fluid. They were also able to find a combination of flow rates that would not normally create droplets, but digitized with UV light.

While this is a nice feature, it partially relies on the preexisting structure that is capable of emulsification. They next presented a design that does not constrict the fluids. This was also able to cause droplet formation with UV light. It was demonstrated that the presence and absence of UV light resulted in digital and continuous flow. This partially fulfills the desired versatility I discussed earlier. But I think that in addition to being able to generate droplets and streams at whim, it is also advantageous to convert droplets back into a stream. The authors briefly mention this, and it seems that the application of blue light can reverse the switch and produce streams. I think the greatest part of this setup is the ability to change the same unit of fluid between continuous and digital.

But what does this research really get us? Well, nothing at first. This isn't a complete device like Dr. Sam Sia's mchip that can detect HIV. But this will surely be incorporated into a device. It really is a tool that can be applied in different ways along with other tools to create a full device. I'll update you further once this has been incorporated and used further.


Diguet, A., Li, H., Queyriaux, N., Chen, Y., & Baigl, D. (2011). Photoreversible fragmentation of a liquid interface for micro-droplet generation by light actuation Lab on a Chip, 11 (16), 2666-2669 DOI: 10.1039/C1LC20328B

Videos reproduced by permission of Damien Baigl and The Royal Society of Chemistry from Lab Chip, 2011, 11, 2666-2669, DOI: 10.1039/C1LC20328B.